Intro

While some serious hobbyists, who interested in precision measurements do own DC voltage standards such as Fluke 732A/732B, resistance standards like Fluke 5450A, Fluke 742 , very few of them have high-end multi-function calibrators. These instruments are usually cost many thousands of dollars and calibration fees often also at prohibitive price for most of hobby labs. Viable alternative to Datron 4800 series multi-function calibrators (MFC) or modern Fluke 5500A / 5700A / 5720A are old obsolete Fluke 5440B or Datron 4700 series units, which are still available from time to time on secondary market for approachable prices. These calibrators are worthy option if one have multiple 6½-digit+ DMMs and sources, which often require wide variety of stable voltages, currents and resistance levels for calibration purposes.

And since we currently have no less than four 7½-digit Keithley Model 2001, two 8½-digit Keithley 2002, one HP 3458A and few other instruments, getting broken calibrator for repair and future use can be a useful investment. Official calibration by Tektronix service for just one Keithley 2001 is priced at $550 USD tag, making support and maintenance of 1 year interval rather expensive.

So when I saw this “unknown” Time Electronics Model 9823 AC-DC calibrator, and checking specification sheet, it was hard to resist than bid button. Let’s see what we get ourselves into.

Specifications comparison

Discontinued Time Electronics Model 9823 specified to be 10ppm/year accurate, with 6½-digit display, self-test function, and AC frequency ranges up to 20kHz. There are also cheaper Model 9822 (30ppm/year, 6½-digit display) and Model 9821 (100ppm/year, 4½-digit display) units. All three today are discontinued and replaced by Time Electronics 5025C (15ppm/year).

Description

Time Electronics Model 9823

Fluke 5500A

Fluke 5720A

Datron 4808

Voltage ranges, DCV

20mV,200mV,2V, 20V,200V, 1kV

330mV,3.3V,33V,330V,1020V

220mV,2.2V,11V,22V,220V,1100V

100µV,1mV,10mV,100mV,1V,10V,100V,1000V

Accuracy 24 hour, ppm

4+2, 3+2, 1+1,1+1,10+10,10+10

5, 4, 4, 4.5, 4.5

4+0.4,3+0.7,2+2.5,2+4,3+40,4+400

2+1u,2+0.5,1+0.5,2+0.5

Accuracy 90 day, ppm

5+2, 5+2, 5+2,5+2,20+10,20+15

50, 40, 40, 45, 45

6+0.4,3.5+0.7,2.5+2.5,2.5+4,3.5+40,4.5+400

15+1µV / 15+1

Accuracy 1 year, ppm

10+2, 10+2,10+2,10+2,30+10,30+15

60, 50, 50, 55, 55

7.5+0.4,5+0.7,3.5+2.5,3.5+4,5+40,6.5+400

35+5µV / 35+5

Tempco, ppm/°C

4 , 3 , 2, 2, 4, 4

0.4,0.3,0.15,0.2,0.3,0.5

2,2,2,2,1.5,1,1.5,2

Voltage ranges, ACV

20mV,200mV,2V,20V,200V,1kV

33mV,330mV,3.3V,33V,330V,1020V

2.2mV,22mV,220mV,2.2V,22V,220V,1100V

1mV,10mV,100mV,1V,10V,100V,1000V

Accuracy 24 hour, ppm

80+50(<1k),200+200(<2k),500+300(<20k)

1+1,1+1,10+10,10+10

70+4 (<20k) ... 2400+20 (<1M)

170+10(<31Hz), 80+10(<100k)

Accuracy 90 day, ppm

200+50, 600+200, 2500+500

300+60, 250+60, 800+80

Accuracy 1 year, ppm

300+50, 800+200, 4000+500

400+60, 300+60, 0.1%+80

Tempco, ppm/°C

15

10, 6, 10

Current ranges, DCI

200µA,2mA,20mA,200mA,2A,10A

3.3mA,33mA,330mA,2.2A,11A

220µA,2.2mA,22mA,220mA,2.2A

100µA,1mA,10mA,100mA,1A,10A

Accuracy 24 hour, ppm

10+5, 10+5,10+5,10+5,25+20,200+200

35+8, 35+8, 35+8, 45+0.8, 60+25

15+10, 15+15, 30+15

Accuracy 90 day, ppm

30+10,30+10,30+10,30+10,60+30,400+300

40+8, 40+8, 40+8, 50+0.8, 65+25

50+15, 115+20, 150+25

Accuracy 1 year, ppm

50+10,50+10,50+10,50+10,100+30,700+300

50+8, 50+8, 50+8, 60+0.8, 80+25

115+20, 250+30, 300+30

Tempco, ppm/°C

8 , 8 , 8, 8, 15, 30

30, 12, 12, 12, 30, 30

Current ranges, DCA (20Hz-1kHz)

200µA,2mA,20mA,200mA,2A,10A

330µA,3.3mA,33mA,330mA,2.2A,11A

220µA,2.2mA,22mA,220mA,2.2A

100µA,1mA,10mA,100mA,1A,10A

Accuracy 24 hour, ppm

100+30 (<200mA), 200+50, 400+200

Accuracy 90 day, ppm

300+100 (<200mA), 350+100, 700+300

Accuracy 1 year, ppm

400+100 (<200mA), 500+100, 1000+300

Tempco, ppm/°C

20 (<200mA), 30, 50

20, 40, 70, 150

Resistance

10Ω,100Ω,1KΩ,10KΩ,100KΩ,1MΩ,10MΩ

0 to 330MΩ synth

0Ω to 100MΩ in 1.0x and 1.9x steps

10Ω,100Ω,1KΩ,10KΩ,100KΩ,1MΩ,10MΩ,100MΩ

Accuracy 90 day, ppm

20, 10, 8, 8, 8, 20, 50

70ppm – 4000ppm

40µΩ, 80ppm, 9ppm, 7.5ppm, 31ppm, 95ppm

30,6,6,6,6,25,100,125

Accuracy 1 year, ppm

50, 20, 20, 20, 25, 60, 100

120ppm – 5000ppm

40µΩ, 95ppm, 10ppm, 8.5ppm, 40ppm, 100ppm

75,20,20,20,25,60,200,500

Tempco, ppm/°C

5, 4, 3, 3, 3, 3, 5

4,6,2,2,2,2,2,2,2,2,2,2,2.5,3,5,8,12

6,2,2,2,2,6,10,20

Dimensions

3U 19” standard rack-mount, 17kg

4U 19” standard rackmount, 36kg

MSRP (February 2016)

Obsolete unit

$24900

$32000 USD

$15000 USD used

This beast is made in 1988, but was “recently” tested in 2004 by Time Electronics and in March 2006 by Absolute Calibration in UK.

Manuals

Teardown and circuit overview

Calibrator is fitted in standard Eurocard 19” 3U rack, such as this made of aluminum frame panels and extruded bars. Outer black cast frame is easily removable, with unit sliding from front face. Various modules are made using standard 3U card type, installed into frame from rear side.

Backplane interconnection performed via ribbon cable with male connectors spaced to mate with cards. Large 110/220VAC fan located on the side provide cooling for high-current power supply/assembly at front side of calibrator frame. All modules and PCBs connect to 64-way signal bus via DIN 41612 type connector.

Front-panel assembly, A293-C

Front panel easily removed by releasing four captive screws on corners and lifting it from the frame.

Few other 74-series logic chips are in charge of LEDs. Obscure MM74C923N is a 20-Key encoder to convert button actuation into digital code. Buttons are made of good quality tactile micro-switches with lever cap with nice clicky feel. Board have few bodge wires and traces cut. Address decoding from the bus data is done with help of 74LS138 and 7400 logic.

LED displays are made by HP, HDSP-3730.

Main gold-plated copper binding posts are covered with steel shield.

High-voltage 200V and 1000V outputs, as well as high-current 10A have separate output terminals for additional safety. So user will not get dangerous output, unless specifically change output connection to device under test. Also there is 3 second delay and loud beeping will be present once high-voltage/high-current ranges are selected. If output connections are incorrect (for example trying to output current into high-impedance load) front panel display will display error message OP ERROR.

Main firmware of the instrument stored in AM2764A UV-erasable EPROM with label 9823 P23. P23 likely meaning version number here. Right next to it is JEDEC-standard SRAM, MHS MH3-6116L-5 and EXELXLS2816AP-250EEPROM for calibration data storage. I had read dumps (available in firmware section) and tested these chips. They confirmed to be OK.

Input jack near calibration ROM is designed for calibration “key” to allow calibration data write. It’s just 3.5mm jack type, so should be easy to bypass later, when we decide to do instrument calibration.

Secondary H.V. power supply – A248-d

High-voltage divider build using pair of ultra-stable resistors, 100KΩ ±0.1% 5ppm/°C AX400/B wire-wound resistors and 9.9MΩ ±0.5% 3ppm/°C UP500/C. High-value resistor metal case is soldered to board with separate wire, likely to provide guarding for inner resistive element and reduce noise from leakage pickup.
These two resistors divide 0-1000V output down to 0-10V, which is further compared with DAC output voltage to regulate high voltage output.

E-core transformer in center used to convert voltage. Few more parts located near backplane connector, such as operational amplifier OP77 and MC14011 Quad 2?Input NAND Gate. Few relays used for range switching.

Trimpot at board corner is used to adjust high voltage gain of supply unit. There is also a buzzer to alarm user when high-voltage output is operating.

There are two connectors for cables.

3-pin cable wiring connection table:

Pin

Signal name

Wire color/cable

1

?

Red on PE white cable

2

?

White on PE white cable

3

None

No pin, key

4

?

Blue on gray cable

4-pin cable wiring connection table

Pin

Signal name

Wire color/cable

1

?

Red on PE white cable

2

?

White on PE white cable

3

None

No pin, key

4

?

Blue on gray cable

5

?

?

Reference assembly – A406-a

All the critical reference assembly is potted in black epoxy box, so it’s exact type of used voltage reference is still remain to be discovered. According to technical manual, there are two precision, aged zener diodes, running at optimum current. Output from these diodes is averaged and amplified to provide master reference voltage of +10.48576 VDC. There is also precision inverter inside potted assembly to provide -10.48576 VDC. This allows either positive or negative reference application, with help of low-thermal FET switch for analog output operation on either polarity.

Epoxy box pinout:

Pin

Signal name

Purpose

1

+18V

Input power supply, positive

2

-18V

Input power supply, negative

3

V Sense 0V

Voltage sense, system common

4

D/A O/P

D/A module signal from DAC

5

O/P DRIVE

Drive output?

6

V SENSE

Sense line to positive output

7

10/1V

Range selection input, 2/20V

8

V/I

Output selection, DCV or DCI

9

+/-

Polarity selection, pos/neg

10

Power 0V

Isolated ground reference 0V

11

H

Feedback to DAC?

12

REF

Reference output to AC board

13

I SENSE 0V

Current sense, system common

14

BUFFERED 0V

Buffered zero output

15

I Sense +

Current sense, positive output

16

No name

Feedback?

Potted reference-amplifier module accepts ±18V input power supply from backplane connector and provides stable voltage or current reference for rest of the calibrator.

Reference output is fed into two D/A converters, both of them working in sync to form a composite 20-bit DAC. Least significant bits are formed by monolithic Analog Devices AD7534 14-bit DAC output. Six higher extra bits are added by separate D/A assembly board, which is discrete R2R logic using high-stability resistors and potted FET assembly. End result is 20-bit DAC code resolution converted into analog voltage. There is also substitute to Analog Devices DAC, MX7534 made by Maxim Integrated. Chip is still in production, even though it’s cost is over $30 USD.

R-2R resistor ladder consist of high quality, very low temperature coefficient resistors which have been carefully chosen, in resistance value and type, to provide best long-term stability. Value of each resistor is further trimmed with multi-turn pot to provide good linearity. Output of the ladder is buffered and fed back to reference board to be combined with LSBDAC output.

These resistors are used as current shunts for current ranges from 200µA to 2A and related 4-wire Kelvin connection circuitry. Three lower current ranges are switched by a FET switch, while higher current 200mA/2A ranges switched by relays. This module also provide control signal to 10A high-current board, located in front side of the instrument.

Cable CON3 wiring connection table:

Pin

Signal name

Wire color/cable

1

?

Red on PE white cable

2

?

White on PE white cable

3

?

Red on gray cable

4

?

Blue on gray cable

5

?

Black on PE white cable

VR1 perhaps adjust current zero?

Resistance standard assembly – A337-g

This assembly is resistance standard of calibrator. Even without schematics it’s operation is very easy to understand. SN7445NBCD-to-DECIMAL decoder switches on one of eight S4-12V relays, which are connecting one of reference resistors to 4-wire Kelvin output at CON2 terminal block or bypass these resistors for voltage/current output.

Also additional full-wave rectifier provides unregulated 14VDC at 1A, to further provide 5V for relays drive and latching relays.

Pair of 22VRMS windings are rectified to supply ±28VDC and regulated ±18VDC for all the analog circuitry. 11V winding can be cascaded with 22V one to give unregulated ±50VDC for output stage of the calibrator DAC.

High current power amplifier

This board generates an unregulated voltage approximately ±4V to supply high-current source. This voltage is regulated by push-pull stage to provide up to 10Amps of current.

PA board with cables connected:

High-current shunt is formed with four long strands, perhaps from resistive material? According to technical manual, output current sense element is 0.01 Ω with kelvin connection. Voltage generated by sensor (0.1V at 10A, simple Ohm law) is amplified 10 times by a differential amplifier and provided back to Reference assembly sense I/P terminals to close the feedback loop and regulate current.

Overall construction

Firmware dumps

Repair workflow

Power supplies checked, working well. Optocouplers checked, all working, but really with slow rise-times, ~11-25µs. Should not be an issue, but worth to check.

Calibrator does turn on okay, front panel shows 9823 P23 message and operating at first glance normally, all buttons responsive, but output is not correct. Output does not match setting, and jumps in very big steps. E.g. setting 0.00000V on 10V range gives correct zero, but if setting increased to 1.00000V output jumps to 1.3612xxV. 1.10000V setting does not change output, still stays 1.3612xxV and only at 1.70000V setting output changes to 1.8531xxV. Similar situation happens on all other ranges. Current ranges giving OP ERROR message on front panel display, likely short of operation error?

I tried replacing old optocouplers with brand new Vishay ILD74’s but that help nothing, calibrator still had same issues with voltage output being adjusted in large steps only from MSB discrete DAC ladder.

Since all power supplies present and stable, voltage reference have stable output on REF module at -10.342xxx VDC, it does seem to me like digital/control issue. Knowing from teardown and schematics above – output is delivered from composite DAC, with 14-bit AD7534 being in charge for lowest 14-bits. I reseated all ICs, tried replacing OP27 op-amps with LT1007 around DAC on REF module, that did not change anything. ILD74 optocouplers were all checked using separate test jig with use of HP 33120A ARB, Keithley 2304PSU and Tektronix CSA7404 scope.

There is not much else going on reference board module, that quickly made DAC chip itself to main suspect. I ordered a replacement AD7534 from UTSource store (which often have many obsolete/rare parts) to test this theory. New chip arrived 29/March/2016, with date code 38 week 2012 in little box with anti-static bag.

Replacement of DAC chip did not change anything though, so we back to digital debug. To see what is DAC chip receiving, TLA714 Logic analyzer was hooked to data bus, A0,A1 and CS/WR signals at the DAC board.

During change of output setting from FP keypad, processor sends four bytes to DAC. All 8 bits of data bus (D[0:7]) work fine, CS/WR (tried together at board) work fine too, latching data by rising edge, but the A0 and A1 signals stay dead low no matter how many times I tried to adjust setting. According to MX7534 datasheet this makes direct DAC register load. And since last transaction is with databus set to all zero, it sets DAC output to zero every time, not the correct value.

This brings us to processor board. All the interfacing to analog side is done from pair of MC6832PIA controllers. Swapping them on processor board made A0/A1 and CS toggle continuously, while D[0:7] bus stay dead low at all times. Perhaps one of PIAs is dead, as these controllers don’t have any internal ROM to explain difference in operation. They are rather simple I/O and IRQ registers for processor bus. Ordered pair of chips from eBay, will see what that does.

Ok, I got the Motorola 68A21P replacement in the board, let’s see what it brings us. A0/A1 signals are present at the DAC now, so control lines and data bus is working correctly now. Here’s how signaling should look:

And since I wanted to get faster rise/fall times out of optocouplers just in case, 100KΩ pull-up resistors on DAC’s digital lines (ones marked 470K on schematics) were replaced as well with 20KΩ which gives almost 1mA of transistor current. Output I-V amplifier on AD7534 output is Linear LT1007CN8, which was tested and confirmed working on separate test jig.

And here’s our progress:

You can see output on calibrator set to 20.00000 VDC and it’s indeed very close to that measured on three Keithley DMMs around (not calibrated). Next step would be full testing of all ranges and functions, to make sure everything works. Once that is done, calibrator will be fully assembled with all covers to perform few days warmup to get all components settled and calibration will be done, using HP 3458A.

Worth to note, that replacement AD7534JN is broken or counterfeit, as it was not working even on the test jig, and VSS pin was constantly loading it’s supply below -11VDC, while original DAC from Time 9823 have this pin steady at zener’s -14.8VDC.

Here are photos of both sides of fake DAC:

Marking look legit to me, it’s nice quality laser etch, just like I see on some other genuine Analog Devices components. Buyers – always beware expensive components with really low prices ;). It’s often worth to spend 20$USD more and save lot of time wondering if part is good or not.

Currently MFC running 10.0000V output, connected to DMM for logging to check if it’s stable.

Tempco of the output voltage seem to be -1.9ppm/°C which is bit high, but actually fits specifications of ±2ppm/°C. We will be checking later if it’s possible to improve stability and reduce temperature sensitivity. For comparison, industry standard multifunction calibrators, such as Fluke 5700A offering ±0.24ppm/°C at 10VDC, in 10°C to 40°C window (22VDC specification ±0.2ppm/°C + 0.4µV).

Unit is being assembled and getting ready for some tests. You can actually really see how small this calibrator is, compared to other beasts of this instruments type, such as Fluke 55xx/57xx or Datron 4700/4800 series.

Calibration

According to calibration manual, most of its calibration data factors (both zero and full scale) is stored in nonvolatile EEPROM memory chip located on the Processor board. The calibration factors are stored twice in memory and when the unit is switched on the 2 sets are compared to check for corruption. Great idea, I’d say, no this Dallas NVRAM stuff with dying batteries, and that’s 10 years before Keithley 2001/2002 and about same time as HP 3458A were designed. This is something, as well as storing two copies, worth to learn from!

To be fair though, unlike Keithley Model 200x or HP 3458A DMMs, this calibrator still have some of the calibration adjustment done using old-school multi-turn trimmers which are mounted on the circuit boards. While older technical manual recommends use of Solartron 7081/7071 or Datron 1071/1081, latest calibration manual recommends HP 3458A, which I will be using.

Linearity testing

One of important specifications of any arbitrary source and generator is linearity. Accuracy of the output is not constant over the range of calibrator for every specific value setting. And since calibration is performed only on the zero and full-scale outputs, everything in between will depend on linearity error, well as other factors.

Knowing DAC architecture of this calibrator, we can easily come up with smallest possible step in output voltage programming, which is for used 20-bit DAC is equal roughly 1.9 µV (20V range / 220). Now we can perform a test, programming output with aligned values and measuring output with more linear instrument than Time 9823. Such a magic device is well known HP 3458A DMM with linearity specification <0.1ppm.